There are heat treatment processes in practice for the production of lightweight materials from natural or artificial silicates. One of the best known forms of such materials is the expanded perlite used more and more extensively today for its advantageous properties. Upon expansion of the perlite, gas develops in the starting material, but the rate of development of the gas is such that closed gas cells are not evolved; rather the grains explode. For this reason expanded perlite is of an open-pore structure. This gives rise in undesirable properties such that its range of application--though it is very wide--cannot be extended to certain fields. Such undesirable properties include: low mechanical strength and hydroscopicity. On account of these two drawbacks, the expanded perlite can be used in construction and in the construction-material industry only to a limited extent.
Glass-like materials consisting of small or large grains has been required for solving several technical problems in the practice. The applicability of such materials would be even more advantageous, if the grains could be of closed pore structure, i.e. the cells are filled with gas. Materials having such properties can be produced in different ways, but the common characteristic of all the known processes is that they are complicated and expensive and treatment of the surface is difficult. Thus these processes became economical only for special applications. The use of such materials in the construction industry is not profitable and not economical.